Ethanol production from biomass has received significant attention in recent years as a source of alternative fuel or biofuel. Ethanol burns more cleanly than fossil fuels, and can be produced using renewable resources such as grains or other renewable starch-containing biomass. A widely used method of producing ethanol from grains is known as “dry milling,” and usually is practiced using corn in the United States. The dry milling process usually uses the starch in the corn or other renewable biomass to produce ethanol through fermentation, which creates a waste stream comprised of byproducts termed “whole stillage.” Whole stillage may be further separated by centrifuge into products known as “wet distillers grains” (WDG) and “thin stillage.” Despite containing valuable oil, whole stillage and thin stillage usually has been treated as waste or used primarily to supplement animal feed, mostly in the form of distillers dried grains with solubles (DDGS). DDGS typically is produced by evaporating the thin stillage, and recombining the resulting concentrate or syrup with the wet distillers grains, and drying the product to have a low moisture content.
Efforts to recover the valuable oil from stillage have encountered significant obstacles. For example, previous use of flash point hydrocarbon solvents, alcohols or polyhydroxy alcohols as extraction solvents for bio-oils have drawbacks. These compounds, while effective, require high concentrations for bio-oil separation which results in potential safety issues. Regulatory requirements for animal feed also has precluded use of these compounds in bio-oil separation, especially in corn fermentation. Another approach involves attempting to separate the oil directly from the thin stillage before the evaporation stage, such as using a centrifuge. However, spinning the thin stillage at this stage does not produce usable oil, but rather merely creates an undesirable emulsion phase requiring further processing. U.S. Pat. No. 5,250,182 shows the use of multiple filters for removing solids and recovering lactic acid and glycerol from thin stillage without the need for evaporation. Filters, and especially the microfiltration and ultrafiltration types proposed for use in the '182 patent, are susceptible to blockage and thus require monitoring and maintenance for this reason. U.S. Patent Application Publication No. 2007/0238891 shows a method of freeing bound oil present in whole stillage and thin stillage, which involves heating the stillage to a temperature said to be sufficient to at least partially separate oil therefrom for recovery from the stillage. In one embodiment, the '891 publication shows the further step of pressurizing the heated stillage to prevent boiling as a form of pressure cooking the stillage.
Algae are another form of biomass that has great potential as a source of bio-oil. As with fermented grains like corn, a number of challenges previously have existed in recovering usable oil in quantity and quality from algae feedstock. One challenge is recovering the desired hydrocarbon molecules from the algae. An option for recovering hydrocarbon products from algae can be to use a solvent extraction based method. Unfortunately, some solvent based methods require use of an algae source that contains little or no water. Dehydration of an algae source to a sufficient degree to allow for this type of solvent extraction can require a high cost of operation. Alternative solvent extraction methods can allow for extraction from an algae sample that contains water. However, a high cost step usually remains, as the solvent has to be separated from the water, for example by distillation.
The present inventors have recognized that there is a need for safer and more efficient, versatile, and economical processes for recovering bio-oil from biomass, such as stillage or algae or other biomass.